Longevity Increases Not Benefitting Everybody

People are living longer on average than they were in 1970, and those extra years of life are being achieved at lower cost, the researchers, led by Doctor Ryan Hum, say in a study.

However, the costs for an extra year of life among adult males in lower-income countries are rising, Hum and his colleagues say, while the costs for an extra year of life among children worldwide and for adults in high-income countries continues to drop.

The researchers made the discovery when they took the Michaelis-Menten (MM) equation – a well-known mathematical model first used to analyze enzyme kinetics in 1913 – and applied it to adult and child mortality at different incomes. They reasoned that just as chemical catalysts affect enzyme velocity; the public health catalysts react with income to affect life expectancy.

"We noticed the similarity in the curvature and became fascinated with the beauty of the analogy," said Hum. The MM equation is standard curriculum for biochemistry, biology and most chemical engineering undergraduate students and we knew there could be added knowledge that we could decipher purely from the math."

"Over the past few decades, research and development of new technologies (drugs, vaccines, policies) have focused mostly on childhood and infectious disease, with fewer worldwide investments for adult chronic diseases," the researchers suggest. "Increasing coverage of inexpensive health interventions such as immunization, insecticide-treated nets, and case management of childhood infections could be contributing to decline in critical income for child survival."

Hum and his colleagues conclude by recommending that society invest in research and treatment of adult chronic disease, most notably the control of smoking and other risk factors for chronic diseases, and low-cost, widely useful treatments for these diseases.

They came up with a new parameter, critical income, which they define as the level of income needed to achieve half of the maximal overall life expectancy found in high-income countries. For example, in 1970, the critical income for overall life expectancy (in inflation adjusted 2005 dollars) was 1.48 Dollar per day. By the year 2007, the critical income had fallen to 1.21 Dollar per day. In other words, a lower national income is needed to achieve a higher life expectancy now, compared to 40 years ago.

However, that good news is due mostly to improvements in children's health and to increased life expectancy in high-income countries, the researchers say. For adults (aged 15 to 59) in lower-income countries, critical income has actually risen since 1970. In other words, adults in low- and middle-income countries need to have higher incomes on average in order to add an extra year of life. Adult males in these countries are especially affected, though adult females also suffer.